A glass melter refers to a glass furnace that produces glass that has a quality level suitable to manufacture a commercial glass product. A premelter is a glass furnace that completes only one of two quality requirements to make most commercial glasses. The two requirements are melting and fining. Melting is the dissolution of glass raw material particles in a melt to produce glass with no remaining undissolved particles. Fining or refining (used synonymously in the industry) is the elimination of gaseous bubbles in the melt that are commonly referred to in the industry as seeds and blisters. The premelter completes most or all of the melting process but not the refining process. Glass that exits a premelter may pass through another melter or refiner to complete the required level of fining.
Submerged combustion in a glass melting furnace is the introduction of a fuel and an oxidant into a glass melt from the bottom of the melt such that they combust and pass the combustion products up through the melt. One of the most unique and desirable features of submerged combustion glass melting is the low temperature required to achieve a relatively high degree of melting of the raw materials used to produce the glass. It has been shown that it is possible to melt the glass raw materials at temperatures of 1950-2000xc2x0 F. at a rate of 2 tons of melt per square foot of melt surface area, and achieve a melting efficiency of 98-99% (i.e. only 1-2% of unmelted raw materials remaining). This compares to typical melting temperatures of 2750-2900xc2x0 F. for most glasses. In submerged combustion, the unmelted portion was silica (sand) grains and these were reduced in size from their initial state. Melting at this rate and within this temperature range had not been possible by any other known melting technology.
The primary reason that this degree of melting is attainable at such low temperatures is due to the violent mixing action that takes place within the melt as the gases combust and bubble through the melt. The strong shearing action that takes place between the molten glass and the unmelted raw material particles greatly accelerates the melting action.
Another positive feature of submerged combustion is the relatively low rate of wear of the refractories that make up the crown, walls and bottom of the furnace due to the low operating temperature required to melt the glass.
One of the unfavorable features and disadvantages of submerged combustion for most glasses is the high quantity of gaseous bubbles that are entrapped in the glass. These gases are from the combustion products and consist of carbon dioxide and nitrogen if the oxidant is air, and carbon dioxide if the oxidant is high purity oxygen. Water vapor, which is also a component of the combustion gases, is mostly dissolved into the glass. The additional time to fine this glass (i.e., rid the glass of seeds and blisters), negates the benefits of the rapid, low temperature premelting.
A second negative feature of submerged combustion is the extent of agitation that occurs as the bubbles rise up through the glass. The bubbles rise at an explosive rate which results in glass being spewed or flung throughout all portions of the furnace above the glass; i.e. the crown and the breast walls, which may harm the furnace refractory and reduce the useful life of the furnace.
A third negative feature of submerged combustion melters and premelters is the objectionable noise that they may produce. Depending upon several variables, such as the burner design, flame velocity, glass temperature and glass depth, the noise can range from a loud, continual thumping sound as the glass erupts at the melt surface and then flops down, to a loud, high frequency squeal.
Bubbling gases up through the glass melting glass furnaces is not uncommon. It consists of installing one or more tubes, called bubblers, through the bottom of the furnace and passing a gas through the bubblers. The bubblers are usually placed in one or more rows across the width of the furnace. They are not typically placed throughout the bottom, however. The purpose of bubbling is primarily to enhance the glass convection currents in the furnace, i.e., upwelling and turnover of the melt. This will bring hot glass from the top of the melt to the bottom and cold glass from the bottom to the top. This action increases the solution rate of the raw materials in the melt. Air is the gaseous medium most commonly used for bubbling; oxygen is occasionally used. One potential disadvantage of bubbling, like submerged combustion, is an increased quantity of seeds and blisters in the melt if the gas bubbled through the glass is nitrogen (as in the case of bubbling with air), or carbon dioxide.
Direct flame impingement melting is described in U.S. Pat. No. 6,237,369 to LeBlanc et al., which is incorporated herein by reference as if fully written out below. An advantage of melting with one or more burners in the roof of a glass furnace over the raw materials used to produce the glass, is an increased rate of melting for a given size glass furnace. This is accomplished as a result of greater heat transfer into the raw material batch and glass.
The present invention is directed to a method of melting glass raw materials more rapidly than is possible at comparable temperatures conventionally, or at lower than conventional glass melting temperatures. Included in the inventive method are favorable features of different glass melting technologies, namely submerged combustion, increasing the water content in glass, bubbling gases through the glass melt, and direct flame impingement melting, while many of the negative aspects of those technologies are avoided.
By glass melting apparatus, as used herein, is meant either a glass premelter or a glass melter (or glass melting furnace), as described above.
The present invention provides a method for melting glass forming batch material including charging the glass forming batch material to a glass melting apparatus; impinging a flame from the combustion of fuel and oxidant proximate to the surface of the batch material to form a glass melt from the batch material; and, bubbling the glass melt in proximity to the impinging flame with at least one fluid capable of solution in the glass melt. Bubbling with such a fluid can produce a shearing action sufficient to enhance the solution rate of the glass forming batch material relative to the same system without bubbling, but without splashing glass and without significant production of seeds or blisters in the glass melt.
The present invention further provides a method for melting glass forming batch material including:
charging the glass forming batch material to a glass melting apparatus, said glass melting apparatus having at least one wall defining an upstream charging zone and a downstream zone connected to a roof and a floor, wherein at least one batch charger for charging the glass forming batch material is contained in the at least one wall defining the charging zone;
providing at least one oxy-fuel burner in the roof over said batch material;
operating the at least one oxy-fuel burner to impinge a flame from the combustion of fuel and oxidant proximate to the surface of the batch material to form a glass melt from the batch material;
providing spaced apart bubblers in the glass melting apparatus; and,
bubbling the glass melt with at least one fluid capable of solution in the glass melt proximate to the impinging flame.
The present method advantageously proceeds without significant production of seeds or blisters in the glass exiting to a glass fining zone. Further, the fluid is advantageously bubbled at a rate to produce a shearing action sufficient to enhance the solution rate of the glass forming batch material relative to the same system without bubbling, but without splashing glass onto the glass melting apparatus walls or roof.